A typical ac synchronous electrical machine includes a field system mounted on the rotor surrounded by a stator winding mounted on the stator. The stator winding will include a plurality of coils that are located in winding slots formed in a surface of the stator assembly. The rotor provides a rotating magnetic field generated by conventional windings with slip rings or brushless excitation power supply. When operated in a motoring mode, ac power supplied to the stator winding will cause the rotor to rotate and produce shaft torque and mechanical power.
For many applications it is desirable for the electrical machine to increase the speed at a constant power and with no further increase in the stator voltage of the electrical machine. This can be achieved by reducing the rotor flux as the speed increases. A similar field weakening control can be achieved with induction machines.
However, if the rotating field system uses permanent magnets then the rotor flux cannot be reduced in the same way. As the rotor flux remains constant then the stator voltage of the permanent magnet electrical machine will increase as the speed increases. This is not ideal because such a permanent magnet electrical machine will normally need N times the rated insulation voltage and will need a power converter with N times the rated voltage and power of the machine, for an increase of N times the rated speed of the machine.